1. Field of the Invention
The present invention relates to a heat fixing device, and more specifically, it relates to a heat fixing device for fixing a toner image which is formed on a surface of a transfer material such as paper held and moved between a heat-resistant film and a pressure roller by pressurization with the pressure roller and heating with a ceramics heater through the heat-resistant film.
2. Description of the Background Art
In general, an image forming apparatus such as a facsimile, a copying machine or a printer, particularly includes a heat fixing device comprising a ceramics heater that fixes a toner image, which has been formed on a photoreceptor drum, onto a transfer material such as paper by heating and pressurizing the same with a heat roller and a pressure roller, in order to heat-fix the unfixed toner image to a surface of the transfer material. A cylindrical heater is generally employed for fixing such a toner image. FIG. 9 is a model diagram schematically showing the structure of a conventional heat fixing device. As shown in FIG. 9, the heat fixing device comprises a heat roller 25 of aluminum and a pressure roller 8 for coming into pressure contact with the heat roller 25. The cylindrical heat roller 25 is provided therein with a cylindrical heater 20 having a heat source such as a halogen lamp. The heat roller 25 and the pressure roller 8 hold paper 9 provided with a toner image therebetween, thereby fixing the toner image formed on the paper 9. With the heat roller 25, the cylindrical heater 20 rotates along arrow R. The pressure roller 8 also rotates along arrow R. Therefore, the paper 9 held between the heat roller 25 and the pressure roller 8 moves along arrow P.
In the aforementioned case, the cylindrical heater 20 itself rotates to conduct heat to the paper 9 through the heat roller 25, thereby fixing the toner image. Therefore, not only the cylindrical heater 20 but the overall heat roller 25 of aluminum must be heated to a temperature capable of fixing the toner image. Consequently, the heat capacity of the overall heater 20 must be increased, leading to high power consumption.
On the other hand, Japanese Patent Laying-Open Nos. 63-313182 (1988), 1-263679 (1989) and 2-157878 (1990) propose heat fixing devices employing plate heaters having small heat capacities and thin films. FIG. 10 is a model diagram showing a schematic structure of such a heat fixing device employing a plate heater. As shown in FIG. 10, the heat fixing device comprises a heat-resistant resin film 7 consisting of polyimide or the like and a pressure roller 8. The heat-resistant resin film 7 is arranged along a heat roller, to be rotatable. The heat-resistant resin film 7 and the pressure roller 8 rotate along arrows R. Paper 9 provided with a toner image is held between the heat-resistant resin film 7 and the pressure roller 8, to move along arrow P. A plate-type ceramics heater 10 is fixed inside the rotating heat-resistant resin film 7. This ceramics heater 10 comprises an insulating ceramics substrate and a heating element provided thereon. The ceramics heater 10 conducts heat to the paper 9 through the heat-resistant resin film 7. This heat fixes the toner image formed on a surface of the paper 9. Due to the plate shape, the heat capacity of the ceramics heater 10 can be remarkably reduced as compared with a cylindrical heater, whereby power consumption can be reduced.
FIGS. 11A to 11C and 12A to 12C illustrate present mounting structures for the ceramics heater 10 in the heat fixing device shown in FIG. 10. FIG. 11A is a top plan view showing a mounted state of the ceramics heater 10, FIG. 11B is a sectional view taken along the line XIB--XIB in FIG. 11A, and FIG. 11C is an enlarged sectional view showing a part XIC in FIG. 11B. FIG. 12A is a top plan view showing another mounted state of the ceramics heater 10, FIG. 12B is a sectional view taken along the line XIIB--XIIB in FIG. 12A, and FIG. 12C is an enlarged sectional view showing a part XIIC in FIG. 12B.
As shown in FIGS. 11A to 11C, the ceramics heater 10 is supported by a stay 6 of resin serving as a heater base. A plurality of cavities 6b are formed on a surface of the stay 6, to be filled up with adhesives 5. The adhesives 5 fix the ceramics heater 10 to the stay 6.
Referring to FIGS. 12A to 12C, on the other hand, a groove 6c which is larger in width than the ceramics heater 10 is formed on a surface of a stay 6. This groove 6c is provided with two rails 6a. The ceramics heater 10 is carried on these rails 6a. Adhesives 5 are filled in a plurality of portions between the two rails 6a. The adhesives 5 fix the ceramics heater 10 to the stay 6.
In the mounting method shown in FIGS. 11A to 11C, the overall surface of the ceramics heater 10 is in close contact with the surface of the stay 6, except the portions bonded to the stay 6 by the adhesives 5. In the mounting method shown in FIGS. 12A to 12C, on the other hand, the ceramics heater 10 is in close contact with the rails 6a of the stay 6.
The heat-resistant resin film 7 slides between the ceramics heater 10 having the aforementioned structure and the pressure roller 8 having a surface of an elastic body (generally rubber) so that the paper 9 provided with the unfixed toner image is fed into a clearance between the heat-resistant resin film 7 and the pressure roller 8 at a constant rate, whereby the toner image is heat-fixed. In recent years, improvement of the throughput of such a heat fixing device is demanded. While the general paper feed rate is about 4 ppm (4 pages per minute: a rate for feeding four sheets of A4 paper under the Japanese Industrial Standards per minute for heat fixing), a higher feed rate of 8 ppm, 16 ppm or 32 ppm is now required.
In order to provide the same heat capacity to the toner image and to attain the same fixation/adhesion strength under a higher feed rate, it is necessary to increase the time for heating the paper, in a simplified way of thinking. To this end, it is necessary to increase the areas of the heating part and the ceramics heater, i.e., the ceramics substrate. In order to cope with the higher feed rate, further, it is necessary to reduce the time for attaining a uniform temperature in a heat soaking part in the ceramics substrate of the ceramics heater in the warm-up (temperature-rise) stage, while maintaining the uniform heating time in the fixing stage. For the purpose of time reduction in the warm-up stage, the inventors have proposed a substrate material of AlN (aluminum nitride) having higher heat conductivity (at least 80 W/mK) than Al.sub.2 O.sub.3 which is generally employed as the substrate material for a ceramics heater at present. When the substrate material for the ceramics heater is prepared from AlN, the heating element conducts heat to the substrate at an extremely high speed, thereby quickly forming a heat soaking zone on the substrate. It is expected that time reduction in the warm-up stage is thus attained.
In each of the present ceramics heater employing Al.sub.2 O.sub.3 as the substrate material and the future ceramics heater employing AlN, heat from the ceramics heater is not sufficiently conducted to the paper but mainly absorbed by the base or stay for the ceramics heater, through the substrate when the ceramics heater is mounted on the stay in the conventional manner shown in FIGS. 11A to 11C or 12A to 12C. Thus, the heat cannot be efficiently conducted to the paper and the toner image formed thereon. Particularly in the arrangement shown in FIGS. 11A to 11C, the ceramics heater is in close contact with the stay substantially along the overall surface except the bonded portions, and hence a great quantity of the heat from the ceramics heater is absorbed by the stay. Still in the method shown in FIGS. 12A to 12C, the heat is absorbed by the stay in quantity although heat insulation efficiency is improved due to an air layer defined between the rails for serving as a heat insulating layer.